The invention resides in a solar converter for a photovoltaic solar system and an operating method therefore, in particular a solar inverter with a corresponding operating method for an extended insolation value range.
Converter systems for photovoltaic solar plants for connection to a solar energy generator and to convert the DC voltage of the solar generator to a single or multiphase AC voltage are known in various configurations. In a common configuration they include an intermediate circuit, which is to be connected to the solar generator and one or several capacitors for storing energy, possibly a voltage amplifier arranged between the solar generator and the intermediate circuit to bring the voltage of the solar generator to a required higher level, as well as an inverter which is connected to the intermediate circuit and whose output can be connected to an AC voltage network for supplying energy to one or more consumers for supplying energy thereto. Generally electronic converter arrangements with semi- or full bridge circuits are used which include controllable semiconductor switching elements and which can be controlled by a controller and a control arrangement at high frequency so as to generate an output AC current which, with respect to its phase and amplitude, is adapted to a semi-shaped 50 Hz or 60 Hz network voltage. Configurations of solar inverter systems and corresponding operating methods are, for example, described in DE 102 21 592 A1 DE 100 20 537 A1 and DE 10 2005 024 465 A1.
Solar generators for photovoltaic installations include generally several solar modules arranged in series and, if appropriate, parallel circuits, whose solar cells convert incoming light of the sun directly to electrical energy. The inverter then uses this energy to generate the AC current suitable for the network or the consumer. The inverter needs to operate with high efficiency in order to achieve a high yield. These factors are extremely important in view the relatively high investment expenses for the installation and the operation of a photovoltaic plant. Modern inverter systems may achieve an overall efficiency of up to 98% or even more when operating at rated conditions.
In order to achieve the highest possible yield, the solar generator is operated at the so-called Maximum Power Point (MPP) which is that point of the current-voltage-diagram of the solar generator where the highest energy output can be achieved that is where the product of current and voltage is at a maximum. The MPP operating point is not constant, but depends on the light radiation density, the temperature and the type of the solar cells or, respectively, the solar generators.
MPP operating point is adjusted in a solar inverter often by a so-called MPP-tracker which controls the voltage of the solar generator to the needed value. To this end, the MPP-tracker varies, for example, the current withdrawn by a small amount, calculates the respective product of current and voltage and adjusts the current value toward a higher energy output. As a result, the correct energy output adaptation can be provided even with changing insolation conditions at the solar generator.
However, the converters are usually not designed for the whole light radiation range. Often inverters are used which have a design performance which is about 10% below the maximum energy output of the solar generator. The reasons herefor are that smaller converters are less expensive and have a substantially higher efficiency under partial load conditions. With the light radiation values as prevalent in central Europe, a partial load operation in the area of about 10% to 80% of the rated solar generator output is obtained. Therefore the inverters are designed conventionally only for radiation energy values of up to about 1000 w/m2.
However, weather conditions where the insolation values are higher so that the solar generator produces energy, which the conventionally dimensioned inverter can not accommodate, will also occur. For example, with low ambient temperatures and cloud formations, for short periods, enhanced by reflections on the clouds, high insolation values of up to 1400 w/m2 or more can occur which cause maximum peak energy outputs of the solar generator. A study of G. Wirth, M. Zehner, B. Giesler: “Sizing and Operational Experience with MWp-PV-Systems-Lessons Learned for System Design Tasks”, 2009 estimates the energy of these radiation peaks which are in the range of 1000 to 1400 W/m2 to be up to 8% of the annual energy yield.
Generally with such an excess insolation yield, exceeding the performance limit of the converter, the MPP operating point is moved toward a higher voltage. However, because the under-dimensioned inverter can not accommodate the excess energy yield, it keeps the operating point at values corresponding to its performance maximum whenever the maximum energy yield exceeds the capacity of the inverter. As a result, part of the energy the solar generator could deliver is not utilized and is lost.
In order to avoid this, the above-mentioned study of G. Wirth et. al., recommends not to under-dimension the inverter relative to the solar generator, as it is common practice, but rather to over-dimension it, for example to 110% inverter capacity in relation to 100% solar generator energy output. Such a set-up however has several disadvantages: First, as mentioned earlier, the inverter becomes more expensive with increasing size and capacity. Second, under normal operation with partial load, for example at 10% of the generator rated energy output, the conversion efficiency is detrimentally affected. Finally, a network or consumer which could accept the peak energies is often not available.
Based hereon, it is an object of the present invention to provide a solar inverter system and an operating method which overcomes the disadvantages of the known inverter systems and to better utilize the performance peaks of the light radiation. In particular, it is an object of the present invention to provide a inverter system which is suitable for an extended insolation value range without the disadvantages of an inverter over dimensioning. Advantageously, the overall efficiency of the inverter should be increased with varying insolation conditions.